1. Technical Field
Embodiments of the present disclosure are related to circuit assembly, and in particular to the use of conductive and nonconductive adhesive to attach components to a circuit board.
2. Description of the Related Art
Typically, components are attached to circuit boards via solder. However, there are circumstances under which solder is not appropriate, and it becomes necessary to employ alternate means. In many cases, adhesives provide certain advantages for use in mounting components, as explained below with reference to FIG. 1.
FIG. 1 is a perspective view of a camera module 100 mounted to a circuit board 102, such as might be used in a cell phone, or other small camera, according to known art. The camera module 100 includes a housing 104, an image sensor chip 106 mounted to a chip carrier 108, a lens 110, and an autofocus module 112. The chip carrier 108 is mounted to the circuit board 102 by a ball grid array comprising a large plurality of solder connections between the chip carrier and corresponding contact pads of the circuit board. Although not shown in detail, the chip carrier 108, the autofocus module 112, and the housing 104 typically nest tightly together, in order to ensure that the lens 110 and the image sensor chip 106 are correctly positioned with respect to each other.
During the assembly process, the chip carrier is positioned on the circuit board 102 and the solder balls of the ball grid array are melted to reflow and form the solder connections. The housing 104 and the autofocus module 112 are then positioned over the chip carrier 108 and image sensor chip 106. The autofocus module 112 includes a voice-coil motor for moving the lens, and is provided with electric leads 114 for power and control. The electric leads 114 must be coupled to corresponding contact pads 116 of the circuit board 102. However, if solder is employed, there is the danger that some of the solder connections of the chip carrier 108 could be damaged by the heat of the additional solder process. Thus, a conductive adhesive 118 is employed to couple the electric leads 114 to the contact pads 116. At the same time, a nonconductive adhesive 120 is used to attach the housing to the circuit board 102.
In addition to permitting the formation of electrical connections without the degree of heat that would be required for solder joints, adhesive attachments can be formulated to be less brittle than solder. Thus, where some amount of flexion of the circuit board may occur, or repeated stress on a particular component, an adhesive connection may be preferable to a solder connection, because it will be less prone to breaking under such circumstances. Nonconductive adhesive can be used to attach component housings and other structural elements, and can be used over conductive traces or connections without affecting the operation of the circuit.
Most conductive adhesives are a composite of thermosetting epoxy resin and conductive metal particles. The most common metals used are silver, nickel, gold, copper, and indium or tin oxides. Relatively soft metals are preferred because they provide better contact between particles by deforming against each other when the adhesive shrinks during cure. Electricity is transmitted through the adhesive along paths of particles in contact with each other.
In selecting an adhesive, it is important that the particular adhesive is capable of wetting both the surface of the circuit board and the material of the component to be attached, in order to form a strong joint.